The manufacture of items such as photographic films and papers requires the substantially continuous coating of aqueous solutions or dispersions of hydrophilic colloids or similar materials. The substrates onto which the solutions are coated are typically supplied to the coating operation as discrete units, usually referred to as "rolls" of base material or "webs". In order for the coating operation to be substantially continuous, the rolls of base material are joined together (spliced) prior to coating. This splicing operation often results in a discontinuity in the surface to be coated. This discontinuity is often in the form of a piece of adhesive tape or similar material (henceforth referred to as a "splice") used to join the end of one roll of support to the beginning of another. This splice often causes non-uniformity in the coating.
A specific cause of coating non-uniformity due to the splice is the air that is entrained between the coating and the substrate at the trailing edge of the splice as shown in FIG. 1. A plurality of discrete webs 2 and 4, each having a leading edge 6 and a trailing edge 8 with respect to a transport direction T, are connected to form an almost endless web for coating. The trailing edge 8 of the preceding web 2 is connected lengthwise with leading edge 6 of a succeeding web 4 using a splicing tape 10. In the description below the expression "web material" stands for the plurality of webs, which are connected by the splicing tape 10 (see FIG. 1). A coating 12 is applied on top of the transported web material and as the coating 12 passes over a trailing edge 14 of the splicing tape 10, it cannot instantaneously regain contact with the surface 15 of the succeeding web 4. An air layer 16 is entrained between the coating 12 and the surface 15 of the succeeding web 4. The presence of this air layer 16 results in such defects as bubbles and streaks (not shown), the severity of which increase with the length of time necessary for the coating to regain contact with the surface 15 of the succeeding web 4. These bubbles and streaks result in areas of non-uniformity in the coating thickness which make the product unsuitable for sale. Additionally, areas of the web with thicker-than-desired coating will not dry in the same period of time as the uniform portions of the coating 12, requiring an increase in drying capacity to accommodate the thicker area of coating. If drying is not sufficient for the overly thick areas of the coating, contamination of the apparatus occurs, resulting in additional waste and curtailment of production for cleaning. It is therefore highly advantageous to limit the severity and duration of this air entrainment.
An additional problem associated with extended entrainment of air between the coating 12 and the surface 15 of the succeeding web 4 is illustrated in FIG. 2. The two successive webs 2 and 4 spliced together by the splicing tape 10 are supported by a coating roller 16 and coated by means of an applicator 18. As the splice between the two webs 2 and 4 passes through the coating 12 such that air is entrained on the trailing edge of the splice, the coating 12 momentarily loses contact with the surface to be coated. If this condition is allowed to persist, the portion of the coating between the web and the coating applicator 18 lacks the constraint normally provided by the surface of the web. This allows considerable movement within the liquid such that intermittent contact can be made with the applicator 18 in a region 20 adjacent to the location that the coating 12 normally leaves contact with the applicator 18. This intermittent contact can result in the formation of drips or bubbles that will result in a streak in the applied coating 12. This streak can continue for an extended period of time, long past the duration of the original entrainment of air at the splice. Some type of intervention requiring a cessation of coating is normally required to remedy this condition. The resulting waste can be many times greater than that caused by the initial coating disturbance. As such, it is highly desirable to limit the factors which can cause this condition.
Many methods and arrangements have been suggested to prevent the above described coating non-uniformities and resulting problems. Included among these methods or arrangements is the use of splice tape with a tapered trailing edge, treatment of the area immediately following the trailing edge of the splice with a hydrophobic liquid, and the mechanical deformation or coarsening of the web surface.
U.S. Pat. No. 3,531,362 by Boums et al. suggests the use of splice tape with a tapered trailing edge. This is often impractical given the thickness and shape of the tape used for splicing. The tape is often made up of a polymeric or paper substrate with a thickness of approximately 0.001 to 0.002 inches coated with an adhesive of similar thickness. In order to provide true relief from the entrained air condition, both the tape substrate and the tape adhesive would need to have the tapered profile described in the patent. This is extremely difficult to attain and control on an ongoing basis, and is thus impractical in the manufacture of many coated products, especially those associated with the photographic industry.
U.S. Pat. No. 3,531,362 and U.S. Pat. No. 4,269,647 by Verkinderen et al. suggest the use of a fast-drying hydrophobic liquid to reduce the occurrence and severity of splice induced coating non-uniformities. In the first case, the liquid is applied by means of a felt-tipped pen or similar device to the area immediately following the trailing edge of the splice tape. In the second case, it is suggested that application of a similar liquid by means of a felt-tipped marker or spray device is more useful if the splice tape is on the side of the web opposite that which is to be coated. It is very likely that these liquids did, in fact, reduce the non-uniformity in the coatings. The improvement achieved seems to be due to the hydrophobic nature of the coating on the web material.
U.S. Pat. No. 4,024,302 by Takagi et al. suggests mechanically deforming or coarsening the surface of the substrate in the area immediately following the trailing edge of the splice. This is achieved by such means as knurling, embossing, or sanding the surface of the substrate. The roughness so induced reduces the non-uniformity seen in the coating in much the same way as the present invention. It is important to note that it is not necessary to make the surface to be coated hydrophobic in order for the mechanism to work. The disadvantage of this method is that the mechanically deforming of the surface generates debris on the surface of the substrate to be coated. While it may be possible to remove much of this debris, it represents a considerable and undesirable source of contamination in the process. This is especially undesirable in such applications as the photographic industry, as even a small amount of contamination will result in waste. Loose debris is particularly troublesome in that it can easily spread throughout the process, resulting in considerable amounts of contaminated product.